Attempts to transfer data more efficiently than the traditional packet switching methods (e.g., X.25) have led to the development of a packet switching technique called frame relay. A frame relay network provides a number of interconnected nodes which are capable of receiving data frames from other network nodes and forwarding those data frames through to other network nodes to an ultimate destination. Nodes are interconnected by transmission paths, each of which supports one or more virtual circuits. Communication from one user to another can thus be made using the pre-defined network connections of the virtual circuits.
Frame relay networks support two types of virtual circuits: permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). With a PVC service, the virtual circuit endpoints route through the frame relay network and PVC identifiers are determined when a user subscribes to the network. PVCs then are similar to leased lines in the sense that the physical connection is always present; call setup and teardown is implemented via commands issued by a network management system at subscription time. SVCs have been referred to as virtual circuits (VCs) "on demand". VCs on demand can be provided using dial access to a frame relay network, for example, from an ISDN network. For the case of offering frame relay services within the broader context of ISDN, a Specification of the International Telecommunications Union (ITU), Recommendation Q.933, provides a protocol for provisioning a virtual circuit "on demand". (The ITU was formerly known as the International Telegraph and Telephone Consultative Commitee or CCITT.)
According to the Q.933 protocol, a SETUP message is transmitted from the calling party to the network/called party. The Q.933 SETUP message specifies various frame relay connection parameters including the DLCI, bit rate, etc. and protocol parameters such as the frame ack window size, etc. Since this method is designed to operate over the ISDN B-channels or D-channel, the channel number element (required for the ISDN connection) is distinct from the DLCI element (required for the frame relay connection) in the SETUP message and both must be transmitted.
In the case of simple, pre-subscribed frame relay services the Q.933 method is relatively inefficient, given the additional protocol stack and the additional information elements required. It would, therefore, be desireable to have an improved method for providing dial access to pre-subscribed frame relay networks.